(1) Field of the Invention
The present invention relates to a developing device and an image forming apparatus using the device, in particular relating to a developing device using a dual-component developer containing a toner and a magnetic carrier, for use in an image forming apparatus for forming images using the toner based on electrophotography, such as an electrostatic copier, laser printer, facsimile machine or the like, as well as to an image forming apparatus using this device.
(2) Description of the Prior Art
Conventionally, image forming apparatuses based on electrophotography such as copiers, printers, facsimile machines and the like have been known. The image forming apparatus using electrophotography is constructed so as to form an image by forming an electrostatic latent image on the photoreceptor drum (toner image bearer) surface, supplying toner to the photoreceptor drum from a developing device to develop the electrostatic latent image, transferring the toner image formed on photoreceptor drum by development to a sheet of paper or the like, and fixing the toner image onto the sheet by means of a fixing device.
Recently, in the image forming apparatuses capable of reproducing full-color and high-quality images, a dual-component developer (which will be referred to hereinbelow as simply “developer”), which can present excellent charge performance stability, is often used. This developer consists of a toner and a carrier, which are agitated in the developing device and frictionally rubbed with each other to produce appropriately electrified toner.
In the developing device, the electrified toner is supplied to a developer supporting member, e.g., the surface of a developing roller. The toner thus supplied to the developing roller is moved by electrostatic attraction to the electrostatic latent image formed on the photoreceptor drum. Hereby, a toner image based on the electrostatic latent image is formed on the photoreceptor drum.
Further, the image forming apparatus of this kind is demanded to be made compact and operate at high speeds, and it is also necessary to electrify the developer quickly and sufficiently and also convey the developer quickly and smoothly.
For this purpose, in order to disperse supplied toner promptly into the developer and provide the toner with an appropriate amount of charge, an image forming apparatus equipped with a developing device of a circulating mechanism including two developer conveying passages that form a circulative path for conveying the developer and two developer agitators that agitate the developer while conveying the developer in the developer passages has been disclosed in patent document 1 (see Japanese Patent Application Laid-open 2005-24592).
In the developing device of patent document 1, usually, auger screws are used as the developer conveying members for circulatively conveying the developer while agitating the developer inside the developer vessel. However, this configuration has suffered the problem that if the rotational speed of the augers is increased in order to raise the speed of developer conveyance, the developer near the interior wall of the developer conveying passage (the space between the auger screw and the wall) is left behind from the developer that is conveyed by the auger screw, stagnating therein. As a result, the developer's agitation performance lowers, causing the problem of toner concentration unevenness.